There has been an enormous increase in research interest towards the development of organic light-emitting devices (OLEDs) after the first report of double-layer structured OLED devices by Tang and vanSlyke [Tang, C. W.; vanSlyke, S. A. Appl. Phys. Lett. 51, 913 (1987)]. OLEDs have been demonstrated to be attractive candidates for flat panel displays owing to their distinct advantages including low operating voltage, low energy consumption, high brightness, high robustness, color tunability, wide viewing angle, relatively low cost, as well as ease of fabrication onto a variety of substrates.
A typical OLED contains several layers of semiconductor sandwiched between two electrodes. The cathode is composed of a low work function metal alloy deposited by vacuum evaporation, whereas the anode is a transparent conductor such as indium tin oxide (ITO). Upon the application of a DC voltage, holes injected by the ITO electrode and electrons injected by the metal electrode will recombine within the semiconductor to form excitons. Subsequent relaxation of excitons will then result in the generation of electroluminescence (EL).
Over the last two decades, an increasing attention has been drawn towards the use of phosphorescent materials, especially transition metal compounds, for the fabrication of OLEDs. In the presence of a heavy metal center, the chance of spin-orbit coupling can be greatly enhanced to facilitate the mixing of singlet and triplet excited states. This yields to a four-fold increase on the internal quantum efficiency of up to 100%. In order to obtain higher EL efficiencies, the use of heavy metal compounds in OLEDs is preferred over purely organic materials, in which the lowest energy excited state of an organometallic compound is commonly a metal-to-ligand charge transfer (MLCT) triplet state, mixed with the excited singlet state through L-S coupling [Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Pure Appl. Chem. 71, 2095 (1999)]. In 1998, Baldo et al. demonstrated a phosphorescent EL device with high quantum efficiency by using platinum(II) 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine (PtOEP) as the dopant. In a multilayer device using tris(8-hydroxyquinoline) aluminium (Alq3) doped with varying concentrations of PtOEP in the light-emitting layer, a strong emission at 650 nm attributed to the triplet excitons of PtOEP was observed [Baldo, M. A.; O'Brien, D. F.; You, Y.; Shoustikow, A.; Sibley, S.; Thompson, M. E.; Forrest, S. R. Nature 395, 151 (1998)]. Later, Baldo et al. also reported the use of fac-tri(2-phenylpyridine)iridium(III) [Ir(ppy)3] as phosphorescent emitting material which was doped in 4,4′-N,N′-diarbazole-biphenyl (CBP) as a host in an OLED to give high quantum efficiency [Baldo, M. A.; Lamansky, S.; Burrows, P. E.; Thompson, M. E.; Forrest, S. R. Appl. Phys. Lett. 75, 4 (1999)].
To date, although much work has been reported on electrophosphorescent materials based on transition metal compounds, most of them have been focused on the use of heavy metal centers such as iridium(III), platinum(II) and ruthenium(II), whereas the use of other metal centers have been much less explored. In particular, in contrast to the isoelectronic platinum(II) compounds which are known to show rich luminescence properties, very few examples of luminescent gold(III) compounds have been reported, probably due to the presence of low-energy d-d ligand field (LF) states and the electrophilicity of the gold(III) metal center. One way to enhance luminescence of gold(III) compounds is through the introduction of strong σ-donating ligands, which was first demonstrated by Yam et al. in which stable gold(III) aryl compounds were synthesized and found to display interesting photoluminescence properties even at room temperature [Yam, V. W. W.; Choi, S. W. K.; Lai, T. F.; Lee, W. K. J. Chem. Soc., Dalton Trans. 1001 (1993)]. Yam et al. later synthesized a series of bis-cyclometalated alkynylgold(III) compounds using various strong σ-donating alkynyl ligands to yield compounds with rich luminescence at both room and low temperatures in various media [Yam, V. W.-W.; Wong, K. M.-C.; Hung, L.-L.; Zhu, N. Angew. Chem. Int. Ed. 44, 3107 (2005); Wong, K. M.-C.; Hung, L.-L.; Lam, W. H.; Zhu, N.; Yam, V. W.-W. J. Am. Chem. Soc. 129, 4350 (2007); Wong, K. M.-C.; Zhu, X.; Hung, L.-L.; Zhu, N.; Yam, V. W.-W.; Kwok, H. S. Chem. Commun.2906 (2005)].